1. Field of the Invention
The present invention relates to a solvent condenser arrangement for a solvent recovery apparatus.
2. Description of the Prior Art
In many industrial processes, for instance, in the drying of articles such as audio tapes or video tapes, solvent vapors are formed which have to be collected and recovered so that they do not enter the atmosphere but can be recycled. Moreover, in such a recovery apparatus usually the inert gas, in general, nitrogen is processed, which serves the purpose of screening the manufacturing apparatus, where the solvent vapors are formed, against the ambient air. In order to prevent such solvent vapors from leaving this apparatus all openings, in general, the inlet and the outlet of this apparatus are provided with transfer chambers which are constantly flushed with the inert gas, in particular, nitrogen to avoid any contact of the atmosphere in the apparatus with the outside air and thus, in particular, emission of the solvent vapors.
Hence, a modern solvent recovery apparatus usually comprises two circuits partially independent of each other, i.e.
an inert gas circuit from which the solvent is condensed, and PA1 a cryogenic part which supplies the transfer chambers of the manufacturing apparatus, for instance, a drier.
The basic construction of such a recovery apparatus for the supply of a drier can be seen from FIG. 1. The drier shown there is continuously or charge-wise charged with the articles to be dried, e.g. video tapes or audio tapes and is provided with transfer chambers 1a, 1b at its inlet and outlet, which are supplied with nitrogen and therefore form a protective curtain between the solvent atmosphere in the inside of the drier 1 and the ambient air.
The solvent-charged nitrogen present in the drier 1 is supplied to a heat exchanger 2 of the solvent recovery apparatus where this gas mixture is precooled. High-boiling components are condensed in the heat exchanger 2 in accordance with the vapor pressure curve. Moreover, the heat exchanger 2 serves the purpose of heat recovery.
The precooled solvent/nitrogen mixture not containing the high-boiling components is supplied from the heat exchanger 2 into a condenser, in which the main portion of the solvent is condensed against external refrigeration. The latter is supplied by the vaporizer 3 of a refrigerating machine 4.
From the vaporizer 3, which also serves as condenser, the two-phase mixture consisting of liquid solvent/purified nitrogen is supplied into a solvent separator 5, where the solvent is separated from the nitrogen; the latter flows from the separator 5 back into the heat exchanger 2 and is heated there in heat exchange with the gas mixture consisting of solvent/nitrogen drained from the drier 1. A fan 6 sucks off the nitrogen from the heat exchanger 2 and returns it into the drier 1.
The recovered solvent present in the separator 5 is intermediately stored in a container 7 and usually repumped to the storage tank for the solvent by means of a pump (not shown).
To prevent oxygen from penetrating into the drier 1 and the solvent from leaving the drier 1 nitrogen is supplied to the transfer chambers 1a, 1b of the drier to produce a positive flow in both directions, namely to the outside of the transfer chambers (the ambient air) and to the inside towards the drier 1.
The flushing nitrogen required therefor is taken partially from the recirculation nitrogen and partially from a tank 8. To guarantee a positive flow from the transfer chambers 1a, 1b into the drier 1 nitrogen is permanently withdrawn from the nitrogen circuit downstream of the separator 5; in accordance with the vapor pressure curve this withdrawn nitrogen, however, still contains so much solvent that this gas mixture may not yet be dissipated into the environment. For this purpose, the gas mixture is heavily supercooled in a cryogenic apparatus 9 and thus purified of the solvent to such a degree until the solvent portion of this gas mixture lies below the regulatory values, so that it is suited for the supply of the transfer chambers. The refrigeration necessary for this purpose is taken from the nitrogen which is used in addition for the flushing of the transfer chambers.
After heating in another heat exchanger 10 the nitrogen passes from the cryogenic apparatus to the transfer chambers 1a, 1b.
In such a solvent recovery apparatus frequently temperatures of -25.degree. C. and below are necessary to condense the solvents used. In this connection, difficulties may arise in the vaporizer 3 of the refrigerating machine, which vaporizer serves as solvent condenser, if a solvent having a high melting point or a solvent mixture comprising components having high melting points have to be condensed. This leads to the sublimation of the components with high melting points and thus to closing of the solvent circuit in the vaporizer 3. The solvent very frequently contains water as well, which sublimes on the cold surfaces of the vaporizer 3 and thus leads to icing of the vaporizer 3.